Role of helix 8 of the thyrotropin-releasing hormone receptor in phosphorylation by G protein-coupled receptor kinase

Biochemical and physiological insights into TRH receptor-mediated signaling

Thyrotropin-releasing hormone (TRH) is an important endocrine agent that regulates the function of cells in the anterior pituitary and the central and peripheral nervous systems. By controlling the synthesis and release of thyroid hormones, TRH affects many physiological functions, including energy homeostasis. This hormone exerts its effects through G protein-coupled TRH receptors, which signal primarily through G_q/11 but may also utilize other G protein classes under certain conditions. Because of the potential therapeutic benefit, considerable attention has been devoted to the synthesis of new TRH analogs that may have some advantageous properties compared with TRH. In this context, it may be interesting to consider the phenomenon of biased agonism and signaling at the TRH receptor. This possibility is supported by some recent findings. Although knowledge about the mechanisms of TRH receptor-mediated signaling has increased steadily over the past decades, there are still many unanswered questions, particularly about the molecular details of post-receptor signaling. In this review, we summarize what has been learned to date about TRH receptor-mediated signaling, including some previously undiscussed information, and point to future directions in TRH research that may offer new insights into the molecular mechanisms of TRH receptor-triggered actions and possible ways to modulate TRH receptor-mediated signaling.

Role of the Helix-8 and C-Terminal Tail in Regulating Proteinase Activated Receptor 2 Signaling

The C-terminal tail of G-protein-coupled receptors (GPCR) contain important regulatory sites that enable interaction with intracellular signaling effectors. Here we examine the relative contribution of the C-tail serine/threonine phosphorylation sites (Ser^383-385, Ser³⁸⁷-Thr³⁹²) and the helix-8 palmitoylation site (Cys³⁶¹) in signaling regulation downstream of the proteolytically activated GPCR, PAR2. We examined Gα_q/11-coupled calcium signaling, β-arrestin-1/-2 recruitment, and MAPK activation (p44/42 phosphorylation) by wild-type and mutant receptors expressed in a CRISPR/Cas9 PAR2-knockout HEK-293 cell background with both peptide stimulation of the receptor (SLIGRL-NH₂) as well as activation with its endogenous trypsin revealed a tethered ligand. We find that alanine substitution of the membrane proximal serine residues (Ser^383-385Ala) had no effect on SLIGRL-NH₂- or trypsin-stimulated β-arrestin recruitment. In contrast, alanine substitutions in the Ser³⁸⁷-Thr³⁹² cluster resulted in a large (∼50%) decrease in β-arrestin-1/-2 recruitment triggered by the activating peptide, SLIGRL-NH₂, but was without an effect on trypsin-activated β-arrestin-1/-2 recruitment. Additionally, we find that alanine substitution of the helix-8 cysteine residue (Cys³⁶¹Ala) led to a large decrease in both Gα_q/11 coupling and β-arrestin-1/-2 recruitment to PAR2. Furthermore, we show that Gα_q/11 inhibition with YM254890, inhibited ERK phosphorylation by PAR2 agonists, while genetic deletion of β-arrestin-1/-2 by CRISPR/Cas9 enhanced MAPK activation. Knockout of β-arrestins also enhanced Gα_q/11-mediated calcium signaling. In line with these findings, a C-tail serine/threonine mutant that has decreased β-arrestin recruitment also showed enhanced ERK activation. Thus, our studies point to multiple mechanisms that regulate β-arrestin interaction with PAR2 and highlight differences in regulation of tethered-ligand- and peptide-mediated activation of this receptor.

JC Polyomavirus Entry by Clathrin-Mediated Endocytosis Is Driven by β-Arrestin

JC polyomavirus (JCPyV) establishes a persistent, lifelong, asymptomatic infection within the kidney of the majority of the human population. Under conditions of severe immunosuppression or immune modulation, JCPyV can reactivate in the central nervous system (CNS) and cause progressive multifocal leukoencephalopathy (PML), a fatal demyelinating disease. Initiation of infection is mediated through viral attachment to α2,6-sialic acid-containing lactoseries tetrasaccharide c (LSTc) on the surface of host cells. JCPyV internalization is dependent on serotonin 5-hydroxytryptamine subfamily 2 receptors (5-HT₂Rs), and entry is thought to occur by clathrin-mediated endocytosis (CME). However, the JCPyV entry process and the cellular factors involved in viral internalization remain poorly understood. Treatment of cells with small-molecule chemical inhibitors and RNA interference of 5-HT₂R endocytic machinery, including β-arrestin, clathrin, AP2, and dynamin, significantly reduced JCPyV infection. However, infectivity of the polyomavirus simian virus 40 (SV40) was not affected by CME-specific treatments. Inhibition of clathrin or β-arrestin specifically reduced JCPyV internalization but did not affect viral attachment. Furthermore, mutagenesis of a β-arrestin binding domain (Ala-Ser-Lys) within the intracellular C terminus of 5-HT_2AR severely diminished internalization and infection, suggesting that β-arrestin interactions with 5-HT_2AR are critical for JCPyV infection and entry. These conclusions illuminate key host factors that regulate clathrin-mediated endocytosis of JCPyV, which is necessary for viral internalization and productive infection.IMPORTANCE Viruses usurp cellular factors to invade host cells. Activation and utilization of these proteins upon initiation of viral infection are therefore required for productive infection and resultant viral disease. The majority of healthy individuals are asymptomatically infected by JC polyomavirus (JCPyV), but if the host immune system is compromised, JCPyV can cause progressive multifocal leukoencephalopathy (PML), a rare, fatal, demyelinating disease. Individuals infected with HIV or taking prolonged immunomodulatory therapies have a heightened risk for developing PML. The cellular proteins and pathways utilized by JCPyV to mediate viral entry are poorly understood. Our findings further characterize how JCPyV utilizes the clathrin-mediated endocytosis pathway to invade host cells. We have identified specific components of this pathway that are necessary for the viral entry process and infection. Collectively, the conclusions increase our understanding of JCPyV infection and pathogenesis and may contribute to the future development of novel therapeutic strategies for PML.

Arginine 313 of the putative 8th helix mediates Gαq/14 coupling of human CC chemokine receptors CCR2a and CCR2b

In man, two CC chemokine receptor isoforms, CCR2a and CCR2b, are present that belong to the rhodopsin-like G protein-coupled receptor family, and couple to G_i and G_q family members. The CCR2 receptors are known to regulate canonical functions of chemokines such as directed migration of leukocytes, and to potentially control non-canonical functions such as differentiation, proliferation, and gene transcription of immune and non-immune cells. We recently reported on the activation of phospholipase C isoenzymes and RhoA GTPases by coupling of the two CCR2 receptors to members of the G_q family, in particular Gα_q and Gα₁₄. So far little is known about the structural requirements for the CCR2/G_q/14 interaction. Interestingly, the CCR2 receptor isoforms are identical up to arginine 313 (R³¹³) that is part of the putative 8th helix in CCR2 receptors, and the 8th helix has been implicated in the interaction of rhodopsin-like G protein-coupled receptors with Gα_q. In the present work we describe that the 8th helix of both CCR2a and CCR2b is critically involved in selectively activating Gα_q/14-regulated signaling. Refined analysis using various CCR2a and CCR2b mutants and analyzing their cellular signaling, e.g. ligand-dependent (i) activation of phospholipase C isoenzymes, (ii) stimulation of serum response factor-mediated gene transcription, (iii) activation of mitogen-activated protein kinases, (iv) internalization, and (v) changes in intracellular calcium concentrations, identified arginine 313 within the amino terminal portion of helix 8 to play a role for the agonist-mediated conformational changes and the formation of a Gα_q/14 binding surface. We show that R³¹³ determines Gα_q/14 protein-dependent but not G_i protein-dependent cellular signaling, and plays no role in G_q/G_i-independent receptor internalization, indicating a role of R³¹³ in biased signaling of CCR2 receptors.

TRH receptor mobility in the plasma membrane is strongly affected by agonist binding and by interaction with some cognate signaling proteins

OBJECTIVES

Extensive research has been dedicated to elucidating the mechanisms of signal transduction through different G protein-coupled receptors (GPCRs). However, relatively little is known about the regulation of receptor movement within the cell membrane upon ligand binding. In this study we focused our attention on the thyrotropin-releasing hormone (TRH) receptor that typically couples to G_q/11 proteins.

METHODS

We monitored receptor diffusion in the plasma membrane of HEK293 cells stably expressing yellow fluorescent protein (YFP)-tagged TRH receptor (TRHR-YFP) by fluorescence recovery after photobleaching (FRAP).

RESULTS

FRAP analysis indicated that the lateral movement of the TRH receptor was markedly reduced upon TRH binding as the value of its diffusion coefficient fell down by 55%. This effect was prevented by the addition of the TRH receptor antagonist midazolam. We also found that siRNA-mediated knockdown of G_q/11α, Gβ, β-arrestin2 and phospholipase Cβ1, but not of G_iα1, β-arrestin1 or G protein-coupled receptor kinase 2, resulted in a significant decrease in the rate of TRHR-YFP diffusion, indicating the involvement of the former proteins in the regulation of TRH receptor behavior. The observed partial reduction of the TRHR-YFP mobile fraction caused by down-regulation of G_iα1 and β-arrestin1 suggests that these proteins may also play distinct roles in THR receptor-mediated signaling.

CONCLUSION

These results demonstrate for the first time that not only agonist binding but also abundance of some signaling proteins may strongly affect TRH receptor dynamics in the plasma membrane.

Functional Role of the C-Terminal Amphipathic Helix 8 of Olfactory Receptors and Other G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) transduce various extracellular signals, such as neurotransmitters, hormones, light, and odorous chemicals, into intracellular signals via G protein activation during neurological, cardiovascular, sensory and reproductive signaling. Common and unique features of interactions between GPCRs and specific G proteins are important for structure-based design of drugs in order to treat GPCR-related diseases. Atomic resolution structures of GPCR complexes with G proteins have revealed shared and extensive interactions between the conserved DRY motif and other residues in transmembrane domains 3 (TM3), 5 and 6, and the target G protein C-terminal region. However, the initial interactions formed between GPCRs and their specific G proteins remain unclear. Alanine scanning mutagenesis of the murine olfactory receptor S6 (mOR-S6) indicated that the N-terminal acidic residue of helix 8 of mOR-S6 is responsible for initial transient and specific interactions with chimeric Gα_{15_olf}, resulting in a response that is 2.2-fold more rapid and 1.7-fold more robust than the interaction with Gα₁₅. Our mutagenesis analysis indicates that the hydrophobic core buried between helix 8 and TM1–2 of mOR-S6 is important for the activation of both Gα_{15_olf} and Gα₁₅. This review focuses on the functional role of the C-terminal amphipathic helix 8 based on several recent GPCR studies.

Deficiency or inhibition of lysophosphatidic acid receptor 1 protects against hyperoxia-induced lung injury in neonatal rats

AIM

Blocking of lysophosphatidic acid (LPA) receptor (LPAR) 1 may be a novel therapeutic option for bronchopulmonary dysplasia (BPD) by preventing the LPAR1-mediated adverse effects of its ligand (LPA), consisting of lung inflammation, pulmonary arterial hypertension (PAH) and fibrosis.

METHODS

In Wistar rats with experimental BPD, induced by continuous exposure to 100% oxygen for 10 days, we determined the beneficial effects of LPAR1 deficiency in neonatal rats with a missense mutation in cytoplasmic helix 8 of LPAR1 and of LPAR1 and -3 blocking with Ki16425. Parameters investigated included survival, lung and heart histopathology, fibrin and collagen deposition, vascular leakage and differential mRNA expression in the lungs of key genes involved in LPA signalling and BPD pathogenesis.

RESULTS

LPAR1-mutant rats were protected against experimental BPD and mortality with reduced alveolar septal thickness, lung inflammation (reduced influx of macrophages and neutrophils, and CINC1 expression) and collagen III deposition. However, LPAR1-mutant rats were not protected against alveolar enlargement, increased medial wall thickness of small arterioles, fibrin deposition and vascular alveolar leakage. Treatment of experimental BPD with Ki16425 confirmed the data observed in LPAR1-mutant rats, but did not reduce the pulmonary influx of neutrophils, CINC1 expression and mortality in rats with experimental BPD. In addition, Ki16425 treatment protected against PAH and right ventricular hypertrophy.

CONCLUSION

LPAR1 deficiency attenuates pulmonary injury by reducing pulmonary inflammation and fibrosis, thereby reducing mortality, but does not affect alveolar and vascular development and, unlike Ki16425 treatment, does not prevent PAH in neonatal rats with experimental BPD.

Structure and function of serotonin G protein-coupled receptors

Serotonin receptors are prevalent throughout the nervous system and the periphery, and remain one of the most lucrative and promising drug discovery targets for disorders ranging from migraine headaches to neuropsychiatric disorders such as schizophrenia and depression. There are 14 distinct serotonin receptors, of which 13 are G protein-coupled receptors (GPCRs), which are targets for approximately 40% of the approved medicines. Recent crystallographic and biochemical evidence has provided a converging understanding of the basic structure and functional mechanics of GPCR activation. Currently, two GPCR crystal structures exist for the serotonin family, the 5-HT1B and 5-HT2B receptor, with the antimigraine and valvulopathic drug ergotamine bound. The first serotonin crystal structures not only provide the first evidence of serotonin receptor topography but also provide mechanistic explanations into functional selectivity or biased agonism. This review will detail the findings of these crystal structures from a molecular and mutagenesis perspective for driving rational drug design for novel therapeutics incorporating biased signaling.

Molecular basis of agonist docking in a human GPR103 homology model by site-directed mutagenesis and structure-activity relationship studies

BACKGROUND AND PURPOSE

The neuropeptide 26RFa and its cognate receptor GPR103 are involved in the control of food intake and bone mineralization. Here, we have tested, experimentally, the predicted ligand-receptor interactions by site-directed mutagenesis of GPR103 and designed point-substituted 26RFa analogues.

EXPERIMENTAL APPROACH

Using the X-ray structure of the β2 -adrenoceptor, a 3-D molecular model of GPR103 has been built. The bioactive C-terminal octapeptide 26RFa(19-26) , KGGFSFRF-NH2 , was docked in this GPR103 model and the ligand-receptor complex was submitted to energy minimization.

KEY RESULTS

In the most stable complex, the Phe-Arg-Phe-NH2 part was oriented inside the receptor cavity, whereas the N-terminal Lys residue remained outside. A strong intermolecular interaction was predicted between the Arg(25) residue of 26RFa and the Gln(125) residue located in the third transmembrane helix of GPR103. To confirm this interaction experimentally, we tested the ability of 26RFa and Arg-modified 26RFa analogues to activate the wild-type and the Q125A mutant receptors transiently expressed in CHO cells. 26RFa (10(-6)  M) enhanced [Ca(2+) ]i in wild-type GPR103-transfected cells, but failed to increase [Ca(2+) ]i in Q125A mutant receptor-expressing cells. Moreover, asymmetric dimethylation of the side chain of arginine led to a 26RFa analogue, [ADMA(25) ]26RFa(20-26) , that was unable to activate the wild-type GPR103, but antagonized 26RFa-evoked [Ca(2+) ]i increase.

CONCLUSION AND IMPLICATIONS

Altogether, these data provide strong evidence for a functional interaction between the Arg(25) residue of 26RFa and the Gln(125) residue of GPR103 upon ligand-receptor activation, which can be exploited for the rational design of potent GPR103 agonists and antagonists.

Fine-tuning somatostatin receptor signalling by agonist-selective phosphorylation and dephosphorylation: IUPHAR Review 5

The biological actions of somatostatin are mediated by a family of five GPCRs, named sst1 to sst5 . Somatostatin receptors exhibit equally high-binding affinities to their natural ligand somatostatin-14 and largely overlapping distributions. The overexpression of somatostatin receptors in human tumours is the molecular basis for diagnostic and therapeutic application of the stable somatostatin analogues octreotide, lanreotide and pasireotide. The efficiency of somatostatin receptor signalling is tightly regulated and ultimately limited by the coordinated phosphorylation and dephosphorylation of intracellular carboxyl-terminal serine and threonine residues. Here, we review and discuss recent progress in the generation and application of phosphosite-specific antibodies for human sst2 and sst5 receptors. These phosphosite-specific antibodies are unique tools to monitor the spatial and temporal dynamics of receptors phosphorylation and dephosphorylation. Using a combined approach of phosphosite-specific antibodies and siRNA knock-down screening, relevant kinases and phosphatases were identified. Emerging evidence suggests distinct mechanisms of agonist-selective fine-tuning for individual somatostatin receptors. The recently uncovered differences in phosphorylation and dephosphorylation of these receptors may hence be of physiological significance in mediating responses to acute, persistent or repeated stimuli in a variety of target tissues.

Dynamic kisspeptin receptor trafficking modulates kisspeptin-mediated calcium signaling

Kisspeptin receptor (KISS1R) signaling plays a critical role in the regulation of reproduction. We investigated the role of kisspeptin-stimulated KISS1R internalization, recycling, and degradation in the modulation of KISS1R signaling. Kisspeptin stimulation of Chinese hamster ovary or GT1-7 cells expressing KISS1R resulted in a biphasic increase in intracellular Ca(2+) ([Ca(2+)]i), with a rapid acute increase followed by a more sustained second phase. In contrast, stimulation of the TRH receptor, another Gq/11-coupled receptor, resulted in a much smaller second-phase [Ca(2+)]i response. The KISS1R-mediated second-phase [Ca(2+)]i response was abolished by removal of kisspeptin from cell culture medium. Notably, the second-phase [Ca(2+)]i response was also inhibited by dynasore, brefeldin A, and phenylarsine oxide, which inhibit receptor internalization and recycling, suggesting that KISS1R trafficking contributes to the sustained [Ca(2+)]i response. We further demonstrated that KISS1R undergoes dynamic ligand-dependent and -independent recycling. We next investigated the fate of the internalized kisspeptin-KISS1R complex. Most internalized kisspeptin was released extracellularly in degraded form within 1 hour, suggesting rapid processing of the internalized kisspeptin-KISS1R complex. Using a biotinylation assay, we demonstrated that degradation of cell surface KISS1R was much slower than that of the internalized ligand, suggesting dissociated processing of the internalized kisspeptin-KISS1R complex. Taken together, our results suggest that the sustained calcium response to kisspeptin is dependent on the continued presence of extracellular ligand and is the result of dynamic KISS1R trafficking.

Molecular mechanisms of platelet P2Y(12) receptor regulation

Platelets are critical for haemostasis, however inappropriate activation can lead to the development of arterial thrombosis, which can result in heart attack and stroke. ADP is a key platelet agonist that exerts its actions via stimulation of two surface GPCRs (G-protein-coupled receptors), P2Y(1) and P2Y(12). Similar to most GPCRs, P2Y receptor activity is tightly regulated by a number of complex mechanisms including receptor desensitization, internalization and recycling. In the present article, we review the molecular mechanisms that underlie P2Y(1) and P2Y(12) receptor regulation, with particular emphasis on the structural motifs within the P2Y(12) receptor, which are required to maintain regulatory protein interaction. The implications of these findings for platelet responsiveness are also discussed.

Binding characteristics of [3H]-JSM10292: a new cell membrane-permeant non-peptide bradykinin B2 receptor antagonist

BACKGROUND AND PURPOSE

A (3) H-labelled derivative of the novel small-molecule bradykinin (BK) B(2) receptor antagonist JSM10292 was used to directly study its binding properties to human and animal B(2) receptors in intact cells and to closely define its binding site.

EXPERIMENTAL APPROACH

Equilibrium binding, dissociation and competition studies with various B(2) receptor ligands and [(3) H]-JSM10292 were performed at 4°C and 37°C. The experiments were carried out using HEK293 cells stably (over)expressing wild-type and mutant B(2) receptors of human and animal origin.

KEY RESULTS

[(3) H]-JSM10292 bound to B(2) receptors at 4°C and at 37°C with the same high affinity. Its dissociation strongly depended on the temperature and increased when unlabelled B(2) receptor agonists or antagonists were added. [(3) H]-JSM10292 is cell membrane-permeant and thus also bound to intracellular, active B(2) receptors, as indicated by the different 'nonspecific' binding in the presence of unlabelled JSM10292 or of membrane-impermeant BK. Equilibrium binding curves with [(3) H]-JSM10292 and competition experiments with unlabelled JSM10292 and [(3) H]-BK showed a different affinity profile for the wild-type B(2) receptor in different species (man, cynomolgus, rabbit, mouse, rat, dog, pig, guinea pig). Characterization of B(2) receptor mutants and species orthologues combined with homology modelling, using the CXCR4 as template, suggests that the binding site of JSM10292 is different from that of BK but overlaps with that of MEN16132, another small non-peptide B(2) receptor ligand.

CONCLUSIONS AND IMPLICATIONS

[(3) H]-JSM10292 is a novel, cell membrane-permeant, high-affinity B(2) receptor antagonist that allows direct in detail studies of active, surface and intracellularly located wild-type and mutant B(2) receptors.

Desensitization, trafficking, and resensitization of the pituitary thyrotropin-releasing hormone receptor

The pituitary receptor for thyrotropin-releasing hormone (TRH) is a calcium-mobilizing G protein-coupled receptor (GPCR) that signals through Gq/11, elevating calcium, and activating protein kinase C. TRH receptor signaling is quickly desensitized as a consequence of receptor phosphorylation, arrestin binding, and internalization. Following activation, TRH receptors are phosphorylated at multiple Ser/Thr residues in the cytoplasmic tail. Phosphorylation catalyzed by GPCR kinase 2 (GRK2) takes place rapidly, reaching a maximum within seconds. Arrestins bind to two phosphorylated regions, but only arrestin bound to the proximal region causes desensitization and internalization. Phosphorylation at Thr365 is critical for these responses. TRH receptors internalize in clathrin-coated vesicles with bound arrestin. Following endocytosis, vesicles containing phosphorylated TRH receptors soon merge with rab5-positive vesicles. Over approximately 20 min these form larger endosomes rich in rab4 and rab5, early sorting endosomes. After TRH is removed from the medium, dephosphorylated receptors start to accumulate in rab4-positive, rab5-negative recycling endosomes. The mechanisms responsible for sorting dephosphorylated receptors to recycling endosomes are unknown. TRH receptors from internal pools help repopulate the plasma membrane. Dephosphorylation of TRH receptors begins when TRH is removed from the medium regardless of receptor localization, although dephosphorylation is fastest when the receptor is on the plasma membrane. Protein phosphatase 1 is involved in dephosphorylation but the details of how the enzyme is targeted to the receptor remain obscure. It is likely that future studies will identify biased ligands for the TRH receptor, novel arrestin-dependent signaling pathways, mechanisms responsible for targeting kinases and phosphatases to the receptor, and principles governing receptor trafficking.

β-Subunit of the Ostα-Ostβ organic solute transporter is required not only for heterodimerization and trafficking but also for function

The organic solute transporter, Ost/Slc51, is composed of two distinct proteins that must heterodimerize to generate transport activity, but the role of the individual subunits in mediating transport activity is unknown. The present study identified regions in Ostβ required for heterodimerization with Ostα, trafficking of the Ostα-Ostβ complex to the plasma membrane, and bile acid transport activity in HEK293 cells. Bimolecular fluorescence complementation analysis revealed that a 25-amino acid peptide containing the Ostβ transmembrane (TM) domain heterodimerized with Ostα, although the resulting complex failed to reach the plasma membrane and generate cellular [(3)H]taurocholate transport activity. Deletion of the single TM domain of Ostβ abolished interaction with Ostα, demonstrating that the TM segment is necessary and sufficient for formation of a heteromeric complex with Ostα. Mutation of the highly conserved tryptophan-asparagine sequence within the TM domain of Ostβ to alanines did not prevent cell surface trafficking, but abolished transport activity. Removal of the N-terminal 27 amino acids of Ostβ resulted in a transporter complex that reached the plasma membrane and exhibited transport activity at 30 °C. Complete deletion of the C terminus of Ostβ abolished [(3)H]taurocholate transport activity, but reinsertion of two native arginines immediately C-terminal to the TM domain rescued this defect. These positively charged residues establish the correct N(exo)/C(cyt) topology of the peptide, in accordance with the positive inside rule. Together, the results demonstrate that Ostβ is required for both proper trafficking of Ostα and formation of the functional transport unit, and identify specific residues of Ostβ critical for these processes.

Molecular dynamics simulations reveal specific interactions of post-translational palmitoyl modifications with rhodopsin in membranes

We present a detailed analysis of the behavior of the highly flexible post-translational lipid modifications of rhodopsin from multiple-microsecond all-atom molecular dynamics simulations. Rhodopsin was studied in a realistic membrane environment that includes cholesterol, as well as saturated and polyunsaturated lipids with phosphocholine and phosphoethanolamine headgroups. The simulation reveals striking differences between the palmitoylations at Cys322 and Cys323 as well as between the palmitoyl chains and the neighboring lipids. Notably the palmitoyl group at Cys322 shows considerably greater contact with helix H1 of rhodopsin, yielding frequent chain upturns with longer reorientational correlation times, and relatively low order parameters. While the palmitoylation at Cys323 makes fewer protein contacts and has increased order compared to Cys322, it nevertheless exhibits greater flexibility with smaller order parameters than the stearoyl chains of the surrounding lipids. The dynamical structure of the palmitoylations-as well as their extensive fluctuations-suggests a complex function for the post-translational modifications in rhodopsin and potentially other G protein-coupled receptors, going beyond their role as membrane anchoring elements. Rather, we propose that the palmitoylation at Cys323 has a potential role as a lipid anchor, whereas the palmitoyl-protein interaction observed for Cys322 suggests a more specific interaction that affects the stability of the dark state of rhodopsin.

The role of palmitoylation in signalling, cellular trafficking and plasma membrane localization of protease-activated receptor-2

Protease-activated receptor-2 (PAR2) is a G protein coupled receptor (GPCR) activated by proteolytic cleavage of its amino terminal domain by trypsin-like serine proteases. This irreversible activation mechanism leads to rapid receptor desensitization by internalisation and degradation. We have explored the role of palmitoylation, the post-translational addition of palmitate, in PAR2 signalling, trafficking, cell surface expression and desensitization. Experiments using the palmitoylation inhibitor 2-bromopalmitate indicated that palmitate addition is important in trafficking of PAR2 endogenously expressed by prostate cancer cell lines. This was supported by palmitate labelling using two approaches, which showed that PAR2 stably expressed by CHO-K1 cells is palmitoylated and that palmitoylation occurs on cysteine 361. Palmitoylation is required for optimal PAR2 signalling as Ca²⁺ flux assays indicated that in response to trypsin agonism, palmitoylation deficient PAR2 is ∼9 fold less potent than wildtype receptor with a reduction of about 33% in the maximum signal induced via the mutant receptor. Confocal microscopy, flow cytometry and cell surface biotinylation analyses demonstrated that palmitoylation is required for efficient cell surface expression of PAR2. We also show that receptor palmitoylation occurs within the Golgi apparatus and is required for efficient agonist-induced rab11a-mediated trafficking of PAR2 to the cell surface. Palmitoylation is also required for receptor desensitization, as agonist-induced β-arrestin recruitment and receptor endocytosis and degradation were markedly reduced in CHO-PAR2-C361A cells compared with CHO-PAR2 cells. These data provide new insights on the life cycle of PAR2 and demonstrate that palmitoylation is critical for efficient signalling, trafficking, cell surface localization and degradation of this receptor.

Helix 8 plays a crucial role in bradykinin B(2) receptor trafficking and signaling

Upon activation the human bradykinin B(2) receptor (B(2)R) acts as guanine nucleotide exchange factor for the G proteins G(q/11) and G(i). Thereafter, it gets phosphorylated by G protein-coupled receptor kinases (GRKs) and recruits β-arrestins, which block further G protein activation and promote B(2)R internalization via clathrin-coated pits. As for most G protein-coupled receptors of family A, an intracellular helix 8 after transmembrane domain 7 is also predicted for the B(2)R. We show here that disruption of helix 8 in the B(2)R by either C-terminal truncation or just by mutation of a central amino acid (Lys-315) to a helix-breaking proline resulted in strong reduction of surface expression. Interestingly, this malfunction could be overcome by the addition of the membrane-permeable B(2)R antagonist JSM10292, suggesting that helix 8 has a general role for conformational stabilization that can be accounted for by an appropriate antagonist. Intriguingly, an intact helix 8, but not the C terminus with its phosphorylation sites, was indispensable for receptor sequestration and for interaction of the B(2)R with GRK2/3 and β-arrestin2 as shown by co-immunoprecipitation. Recruitment of β-arrestin1, however, required the presence of the C terminus. Taken together, our results demonstrate that helix 8 of the B(2)R plays a crucial role not only in efficient trafficking to the plasma membrane or the activation of G proteins but also for the interaction of the B(2)R with GRK2/3 and β-arrestins. Additional data obtained with chimera of B(2)R with other G protein-coupled receptors of family A suggest that helix 8 might have similar functions in other GPCRs as well.

Binding between a distal C-terminus fragment of cannabinoid receptor 1 and arrestin-2

Internalization of G-protein-coupled receptors is mediated by phosphorylation of the C-terminus, followed by binding with the cytosolic protein arrestin. To explore structural factors that may play a role in internalization of cannabinoid receptor 1 (CB1), we utilize a phosphorylated peptide derived from the distal C-terminus of CB1 (CB1(5P)(454-473)). Complexes formed between the peptide and human arrestin-2 (wt-arr2(1-418)) were compared to those formed with a truncated arrestin-2 mutant (tr-arr2(1-382)) using isothermal titration calorimetry and nuclear magnetic resonance spectroscopy. The pentaphosphopeptide CB1(5P)(454-473) adopts a helix-loop conformation, whether binding to full-length arrestin-2 or its truncated mutant. This structure is similar to that of a heptaphosphopeptide, mimicking the distal segment of the rhodopsin C-tail (Rh(7P)(330-348)), binding to visual arrestin, suggesting that this adopted structure bears functional significance. Isothermal titration calorimetry (ITC) experiments show that the CB1(5P)(454-473) peptide binds to tr-arr2(1-382) with higher affinity than to the full-length wt-arr2(1-418). As the observed structure of the bound peptides is similar in either case, we attribute the increased affinity to a more exposed binding site on the N-domain of the truncated arrestin construct. The transferred NOE data from the bound phosphopeptides are used to predict a model describing the interaction with arrestin, using the data driven HADDOCK docking program. The truncation of arrestin-2 provides scope for positively charged residues in the polar core of the protein to interact with phosphates present in the loop of the CB1(5P)(454-473) peptide.

Importance of regions outside the cytoplasmic tail of G-protein-coupled receptors for phosphorylation and dephosphorylation

Two GPCRs (G-protein-coupled receptors), TRHR (thyrotropin-releasing hormone receptor) and beta(2)AR (beta(2)-adrenergic receptor), are regulated in distinct manners. Following agonist binding, TRHR undergoes rapid phosphorylation attributable to GRKs (GPCR kinases); beta(2)AR is phosphorylated by both second messenger-activated PKA (protein kinase A) and GRKs with slower kinetics. TRHR co-internalizes with arrestin, whereas beta(2)AR recruits arrestin, but internalizes without it. Both receptors are dephosphorylated following agonist removal, but TRHR is dephosphorylated much more rapidly while it remains at the plasma membrane. We generated chimaeras swapping the C-terminal domains of these receptors to clarify the role of different receptor regions in phosphorylation, internalization and dephosphorylation. beta(2)AR with a TRHR cytoplasmic tail (beta(2)AR-TRHR) and TRHR with a beta(2)AR tail (TRHR-beta(2)AR) signalled to G-proteins normally. beta(2)AR-TRHR was phosphorylated well at the PKA site in the third intracellular loop, but poorly at GRK sites in the tail, whereas TRHR-beta(2)AR was phosphorylated strongly at GRK sites in the tail (Ser(355)/Ser(356) of the beta(2)AR). Both chimaeric receptors exhibited prolonged, but weak, association with arrestin at the plasma membrane, but high-affinity arrestin interactions and extensive co-internalization of receptor with arrestin required a phosphorylated TRHR tail. In contrast, swapping C-terminal domains did not change the rates of phosphorylation and dephosphorylation or the dependence of TRHR dephosphorylation on the length of agonist exposure. Thus the interactions of GPCRs with GRKs and phosphatases are determined not simply by the amino acid sequences of the substrates, but by regions outside the cytoplasmic tails.